I'm a senior designer who often contributes to front-end code when it's convenient for my client.

Fixing and updating the README when I join a new team and set up their dev environment is always extremely well-received.

If i'm gonna untangle something, i may as-well write some notes. If i'm writing notes on it already, i may as-well refine the grammar a bit and update the docs. It's really quite small effort compared to the main work of learning the system, so i don't quite get why so few people do it.